1. Field of the Invention
The present invention relates to an adhesive composition and an adhesive optical component using the adhesive composition. More particularly, the present invention relates to an adhesive composition which, by application to easily hydrolyzable materials such as substrates and adherends, suppresses degradation of the materials by hydrolysis and improves durability of the materials; an adhesive composition which provides excellent stress relaxation without plasticizers, suppresses degradation of easily hydrolyzable materials such as substrates and adherents by hydrolysis by application to the material, suppresses degradation of the composition itself and gives adhesive optical components having excellent quality; and an adhesive optical component comprising the adhesive composition such as a polarizing plate and a plate for phase differentiation.
2. Description of Related Art
As the adhesive, acrylic adhesives, polyurethane adhesives, polyester adhesives, rubber adhesives and silicone adhesives have heretofore been used. Among these adhesives, acrylic adhesives are widely used. An acrylic adhesive contains, in general, a copolymer of (meth)acrylic esters and a crosslinking agent. As the copolymer of (meth)acrylic esters, for example, a copolymer of a (meth)acrylic ester such as butyl (meth)acrylate, 2-ethylhexyl (meth)acrylate, isooctyl (meth)acrylate and decyl (meth)acrylate with a functional monomer for forming crosslinking points is used. The functional monomer is, specifically, a functional monomer having hydroxyl group such as hydroxyethyl (meth)acrylate and hydroxypropyl (meth)acrylate or a functional monomer having carboxyl group such as (meth)acrylic acid, maleic acid, crotonic and itaconic acid.
However, when the acrylic adhesive is applied to a material which is easily hydrolyzed such as films of cellulose acetate, a problem arises in that the material tends to be hydrolyzed due to the effect of carboxyl group in the copolymer of (meth)acrylic esters contained in the acrylic adhesive. The degradation of the material by hydrolysis takes place markedly, in particular, under an environment of a high temperature and high humidity.
Some optical components are used after a polarizing plate is attached to the surface. Typical examples of such optical components include liquid crystal cells in liquid crystal displays (LCD). In general, a liquid crystal cell has a structure in which two transparent electrode substrates having an oriented layer are placed in a manner such that a specific gap is formed between them with a spacer and the oriented layers face to each other at the inside, peripheral portions of the transparent electrode substrates are sealed, a liquid crystalline material is placed and held inside the gap between the transparent electrode substrates and a polarizing plate is disposed on each outer surface of the two transparent electrode substrates via an adhesive layer.
FIG. 1 shows a perspective view exhibiting the structure of an example of the polarizing plate described above. As shown in FIG. 1, the polarizing plate 10 has a substrate having a three-layer structure in which triacetylcellulose (TAC) film I 2 and TAC film II 2′ are laminated on the faces of a polarizing plate 1 made of polyvinyl alcohol. On one face of the substrate, an adhesive layer 3 for sticking the substrate to an optical component such as a liquid crystal cell is formed. A release sheet 4 is attached to the adhesive layer 3. In general, a film for protecting the surface 5 is disposed on the face of the polarizing plate opposite to the face having the adhesive layer 3.
When the above polarizing plate is attached to the above liquid crystal cell, the release sheet 4 is removed first, then the polarizing plate is stuck to the liquid crystal cell via the exposed adhesive layer and the film for protecting the surface 5 is removed.
For the adhesive layer disposed on the polarizing plate, acrylic adhesives are widely used. However, as described above, the acrylic adhesive promotes hydrolysis of the TAC film of the polarizing plate due to the effect of carboxyl group in the copolymer of (meth)acrylic esters and the polarizing plate is degraded. The degradation of the polarizing plate takes place markedly, in particular, under an environment of a high temperature and a high humidity.
In liquid crystal display apparatuses of the STN type, it is widely conducted that a plate for phase differentiation is disposed between a liquid crystal cell and a polarizing plate. As the plate for phase differentiation, in general, a laminate having a TAC film on one or both faces of a stretched film of polyvinyl alcohol and a layer of an acrylic adhesive on the face of the TAC film, is used. An liquid crystal cell is formed by laminating one or a plurality of plates for phase differentiation to an STN cell via the above adhesive layer and then laminating a polarizing plate on the outermost layers. The thus prepared liquid crystal cell also has a problem in that degradation of the TAC film by hydrolysis takes place in the plate for phase differentiation similarly to the degradation of the TAC film in the polarizing plate.
To overcome the above problems, for example, a method in which the amount of carboxyl group contained in the adhesive is decreased (Japanese Patent Application Laid-Open No. Showa 59(1984)-111114) and a method in which a tertiary amine is added (Japanese Patent Application Laid-Open No. Heisei 4(1992)-254803) have been proposed. However, the method in which the amount of carboxyl group is decreased has a drawback in that the excellent balance between the physical properties of the adhesive is inevitably lost and the method in which a tertiary amine is added has a drawback in that the pot life of the adhesive decreases since control of the reactions between a crosslinking agent and various functional groups in the adhesive becomes difficult and workability in various steps deteriorates.
The polarizing plate which is attached to the liquid crystal cell via the adhesive layer has the three-layer structure described above. Due to the properties of the materials, the polarizing plate has poor dimensional stability and, in particular, change in the dimension by contraction or expansion is great in the environment of a high temperature or a high temperature and a high humidity.
However, since, in general, an adhesive having a great adhesive ability is used in the above polarizing plate, stress caused by the change in the dimension of the polarizing plate cannot be absorbed and relaxed by the adhesive layer although lifting and peeling caused by the change in the dimension of the polarizing plate can be suppressed. More specifically, in FIG. 1, TAC film II 2′ at the front face tends to contract or expand due to change in the humidity and the temperature. On the other hand, TAC film I 2 cannot not contract or expand easily since TAC film I is firmly adhered to the liquid crystal cell via the adhesive layer 3 and the adhesive layer cannot flexibly follow the change in the dimension. As the result, ray passing through TAC film I toward TAC film II cannot proceed straight. This causes undesirable phenomena such as leak of light.
To overcome the above problems, heretofore, a plasticizer is added to the adhesive so that the adhesive is flexible to a suitable degree and stress relaxation takes place. However, the adhesive containing a plasticizer has drawbacks in that the plasticizer bleeds out and that the adherend is stained with the plasticizer when the polarizing plate is peeled by the bleeding out. When a polyfunctional crosslinking agent having a functionality of three or greater is used in an adhesive, the number of crosslinks in the adhesive is decreased. However, the holding ability, i.e., the adhesion with the adherend, inevitably decreases in this case and problems such as lifting and peeling of the polarizing plate tend to arise with passage of the time.
Intensive studies on adhesive compositions exhibiting excellent stress relaxation without adding plasticizers have been made by the present inventors to overcome the above problems and it was found that excellent stress relaxation can be exhibited by using a copolymer of (meth)acrylic esters having a great molecular weight and an oligomer of (meth)acrylic esters having a small molecular weight in combination. However, when this adhesive composition is applied to a polarizing plate and the like, it was found that an undesirable phenomenon occasionally took place in that brightness was different at portions around the edges and at other portions of the polarizing plate.
Moreover, a problem takes place in the acrylic adhesive in that the molecular weight of the copolymer of (meth)acrylic esters decrease since degradation takes place under a condition of a high temperature and a high humidity although the degradation proceeds slowly. As the result, cohesive force in the adhesive becomes insufficient and lifting and peeling take place between a substrate such as a polarizing plate and a plate for phase differentiation and an adherend such as a plate of glass or polycarbonate.